Force beans, trimmed ends first (keeping strings to the side), up through funnel of frencher (if using; see cooks' note, below).
Cook beans in 2 batches in a 6- to 8-quart pot of boiling salted water, uncovered, until just tender, 4 to 5 minutes.
Transfer beans with a slotted spoon to a large bowl and toss with salt, pepper, vinegar, and oil.
Serve hot or warm.
If you do not have a frencher, beans can be halved diagonally.Beans can be frenched (but not cooked) 1 day ahead and chilled in a sealed plastic bag lined with dampened paper towels.